He's playing Epic, and most often you don't use Massive damage in Epic because you'd be rolling for each character every hit, and then it really sucks.
Anyway, I had this problem once, so I did a few things:
1. Made an evil divine caster who had an artifact that could sever another divine caster's link with his deity and therefore take his spells away.
2. Took the gods away (ie Dragonlance-esque).
3. Seeing a Druid spell that only True Neutral Druids can cast, that can strip another divine caster of her spells until she attones. (If the good cleric is being too good, or the evil cleric is being to evil they get nuked for a time).
you could also try to implement the VP/WP system from Unearthed Arcana; that helps a TON with situations like that.
"Oh look, Epic Barbar, you took a crit for a 100, looks like you only have 22 WP's. Well, looks like you'r severed at the waist pal."
You could also limit healing spells, or just kill off a few divine caster's and tell the players thier next character can't be a full divine caster.
And then next time you could always send 2 Mithral Golems after them.
or an Atropal
or a lesser god
or have a Time/Space/Reality rift occur, and have some D20 modern or future goons show up with thier size 12's and thier nuclear missiles selling game guys to the population to take over the world, and if they don't work and slave away they don't get new batteries! (Yeah, I have a 2 year old, ~sigh~ gotta love the Disney Channel)
kc
Edit: Just thought of this:
If the divine casters are keeping the barbar alive, start wounding HIS pride and the pride of the casters. Are they his minions? Why are they keeping him alive? Rumors around the city and the tavern are saying he can't do anything by himself. He's only still alive because of caster's Al, Joe, Bob, and Tina.
Also, be sneaky, and say that some god has seen his exploits, and has deemed him worthy of lesser godhood. send him on some solo quests which are next to impossible to accomplish through force of arms. MAKE HIM use his head ie smarts. If you've ever read Conan, you'll see that he is also very smart, and more often than not he uses his brains to get out of scrapes rather than his brawn (though the parts where he uses his brawn are usually pretty incredible, by Crom!)
Design some kind of ha;f-etheral golem that GAINS hp every time he hits it, instead of getting weaker, but make it so the same Golem takes damage by Perform checks (ie ther barbarian starts telling it his life story, and the golem starts smoking and melting and junk).
